Natural methane (CH4) emissions have gained much attention over the past few decades due to the importance of methane as a potent greenhouse gas. Methane's lifetime in the atmosphere is much shorter than carbon dioxide (CO2), but CH4 is more efficient at trapping radiation than CO2 (i.e., pound for pound, the comparative impact of CH4 on climate change is over 20 times greater than CO2 over a 100-year period). Methane is emitted by natural sources such as wetlands, as well as human activities such as leakage from natural gas systems and the raising of livestock. In 2012, CH4 accounted for about 9% of all U.S. greenhouse gas emissions from human activities (http://epa.gov/climatechange/ghgemissions/gases/ch.4.html#content). Of the various sources for natural methane emissions identified, the wood-feeding termite group is arguably the most significant, to the point where termites have been reported to be the largest source of greenhouse gases (methane) emissions on the planet Earth.
Bacterial methanogenesis is a ubiquitous process in most anaerobic environments. There are three major substrates used by methanogens to produce methane: i) CO2, ii) compounds containing a methyl group, or iii) acetate. The association of bacterial methanogenesis with anaerobic decomposition of organic matter in microbial habitats such as the intestinal tract of animals, sewage, sludge digester, muds of various aquatic habitat etc., has been well established. Thus, gas production commonly observed in nature is mainly the result of the growth of methanogens under specific energy sources that were formed as a result of microbial decomposition of organic matter.
Methanogens belong to the domain Archaea. The diversity of archaea found in the rumen of many organisms has been reviewed by many researchers. Most archaea identified in the rumen of animals belong to known methanogen clades with a predominance of Methanobrevibacter spp. The pooled data from several surveys show that the Methanobrevibacter clade accounts for nearly two-thirds of rumen archaea. The remaining one-third was composed, of roughly equal parts by phylotypes belonging to methanomicrobium and the rumen cluster C.
Most rumen methanogens do not contain cytochromes and although they are less efficient at obtaining energy through the production of methane than their cytochrome-containing relatives of the order methanosarcinales, they are better adapted to the environmental conditions prevailing in the rumen. They have a lower threshold for hydrogen (H2) partial pressure, a faster doubling time, that can be as short as 1 h, and they have the potential to develop better at the mesophilic temperature and the near neutral pH of the rumen.
Termites are eusocial insects that belong to the order isoptera and play a major role in tropical ecosystem. Their basic food is plant matter, both living and dead. The main diet of most of the termite species consists of wood, foliage, humus or a mixture of these foods. It is not known whether isopteran have a significant role in rumen methanogenesis but methanogens attached to the gut epithelium have been described in termites, and in such a microaerobic environment they are capable of producing methane and reducing oxygen at the same time.
Termites are divided into two groups, i) lower termites, and ii) higher termites. Lower termites is a group of six evolutionary distinct termite families (the microbial community in the gut of phylogenetically lower termites) comprising both flagellated protists and prokaryotes. This group includes approximately 85% of all termite species that also harbors a dense and diverse population of gut prokaryotes that typically lack eukaryotic flagellated protists. Higher termites secrete their own digestive enzymes and are independent of gut microorganisms in their nutrition. The lower termites also possess this ability, but their production of cellulolytic enzymes is apparently inadequate. Hence, lower termites mostly depend on the activity of gut microorganisms for their nutrition, which are present in the hind gut region. Methanogens play a crucial role in this community of gut microbiota: if methanogens are disrupted or impeded the ecology of the system fails and the host organism will suffer.
Methanogenesis is an important component of microbial carbon metabolism in the hind gut termite digestive system. Methanogenic bacteria share physiological and biochemical characters such as ability to anaerobically oxidize hydrogen and reduce carbon dioxide to methane. One of the most fascinating nutritional symbioses exists between termites and their intestinal microflora: a symbiosis that permits termites to live by xylophagy, or the consumption of wood. The termite gut represents an excellent model of highly structured micro-environments. Apart from its natural role of conversion of woody and cellulosic substances into useful products of termite gut, microbiota contribute significantly to greenhouse gas effect through methane generation.
FIG. 1 illustrates a gut of a termite and reaction chains that are taking place therewithin. The adult termite gut consists of fore gut (which includes the crop and muscular gizzard), the tubular mid gut (which as in other insects is a key site for secretion of digestive enzymes and for absorption of soluble nutrients) and relatively, a voluminous hindgut (which is also a major site for digestion and for absorption of nutrients). The morphological diversity of the termite gut microbiota is remarkable and has been documented for both lower and higher termites. Although some bacteria colonize the foregut and midgut, the bulk of intestinal microbiota is found in the hindgut, especially in the paunch, which is, the region immediately posterior to the enteric valve. The hindgut compartments harbor the bulk of the intestinal microbiota. These tracts were initially considered as ‘fermentation chambers’ analogous to the rumen of sheep and cattle (e.g. anoxic environments for an anaerobic, oxygen-sensitive microbiota).
Researchers have reported that arthropod gut provides a suitable niche for microbial activity, but the nature of microflora and their distribution depended on the physicochemical conditions like pH, redox potential and temperature of that region. Further research supported that the presence of large number of aerobic, facultative and anaerobic microflora showed that hindguts are a purely anoxic environment together with steep axial pH gradients in higher termites. Among the different physiochemical conditions, pH and redox potential are the important factors which determine the type of microflora in the gut, while the pH of the foregut and midgut is around neutrality, whereas the paunch, colon and rectum appear to be slightly acidic.
FIG. 2 identifies known reductive reactions that occur in the gut of the termites. The most important metabolic activities traditionally attributed to the gut microbiota are, first, hydrolysis of cellulose and hemicelluloses, second, fermentation of the depolymerization products to short-chain fatty acids, which are then resorbed by the host, and third, intestinal nitrogen cycling and dinitrogen fixation. In the phylogenetically lower termites, a large fraction of hindgut volume (up to one-third of the body weight of a termite) is occupied by anaerobic flagellates, which phagocytize and degrade the wood particles comminuted by the termite. The phylogenetically higher termites do not harbor flagellates within their gut. Instead, an acquisition of cellulases with the food (in case of the fungus-cultivating termites) or a host origin of the cellulolytic activities has been suggested.
FIG. 3 illustrates a carbohydrate metabolism in wood and litter feeding termites. Termites are good sources of wood degrading enzymes such as cellulase-free xylanase, laccases that are potentially involved in phenolic compounds degradation suitable for paper and pulp industry and glucosidases. The metagenomic analysis of hindgut microbiota of higher termite shows the presence of diverse endoxylanases, endoglucanases, phosphorylases, glucosidases, nitrogenases, enzymes for carbon dioxide reduction and enzymes used in new ways for producing lignocelluloses based biofuels production and acetate production. Daily hydrogen turnover rates were 9-33 m3 H2 per m3 hindgut volume, corresponding with the 22-26% respiratory activity of the termites. This makes H2 the central free intermediate during lignocellulose degradation and the termite gut, with its high rates of reductive acetogenesis, the smallest and most efficient natural bioreactor currently known.
Termites inhabit many different ecological regions, but they are concentrated primarily in tropical grasslands and forests. Symbiotic micro-organisms in the digestive tracts of termites (flagellate protozoa in lower termites and bacteria in higher termites) produce methane. Termites emit large quantities of methane, carbon dioxide and molecular hydrogen into the atmosphere. Significant studies have been performed on diversity, social structure, physiology and ecology of the termites as source of methane contributing to the sources of atmospheric greenhouse gas. Methane production by termites was first reported by Cook (1932) who observed the evolution of a gas from a species of termite.
FIG. 4 illustrates the results of studies showing large variations in amount of methane produced (in a termite's digestive track during the breakdown of cellulose by symbiotic micro-organisms) for different species. Research also found average methane production rates of 0.425 μg CH4/termite/day for the lower termite species and 0.397 μg CH4/termite/day for the higher termite families. Environmental conditions such as light levels, humidity, temperature, as well as carbon dioxide and oxygen presence play a key part in methane production. Termites prefer the absence of solar radiation, an immobile atmosphere, saturated or nearly saturated, relative humidity, high and stable temperatures and even elevated levels of carbon dioxide. Although termite populations are active in the middle latitude environments, the vast concentrations of mounds and nests are found in the lower latitude tropical forests, grasslands and savannahs of Africa, Asia, Australia and South America. It is estimated that these regions contribute approximately 80% of global termite emissions.
Researchers performed laboratory experiments using termite mounds under glass enclosures, with varying diet patterns and temperatures, while all other variables remained stable. It was found that the capacity of termites to produce methane varied from species to species, within groups from different mounds or nests of a particular species. But all species produced methane which indicates that methanogens are active components of their biology. The six different species studied produced methane at rates that ranged over more than two orders of magnitude. Raising the temperature by 5° C. within each species' caused a 30-110% increase in the measured methane emissions. Prior laboratory and field research seems to show that termites preferred temperatures in excess of 10° C. above the ambient air temperatures, determined by their geographical locations. A positive correlation between amounts of biomass consumed and methane emitted was observed, with the average being 3.2 mg CH4 per gram of wood.
Methanogenic bacteria have been associated with protozoa in termites. Though methanogens are generally strict anaerobes, their metabolic responses to the presence of oxygen and their sensitivity to it vary with the species. Methanobacterium sp. was isolated from the termite hindgut. Methanobrevibacter cuticulam and M. curvatus were isolated from the hindgut of the termite Reticulitermes flaviceps. The presence of M. arboriphilicus and Methanobacterium bryantii in the guts of wood eating higher termites has also been reported.
Termite guts are the world's smallest bioreactors. The presence of carbohydrate-fermenting bacteria and protozoa, high levels of volatile fatty acids in the gut fluid and the occurrence of typical anaerobic activities such as homoacetogenesis and methanogenesis resemble the situation encountered in the rumen of sheep and cattle.
Methane is a metabolic end product in the hindgut of most termites. It has been estimated that these insects contribute approximately 2 to 4% to the global emissions of this important greenhouse gas. Methanogenic archaea, which are easily identified by their coenzyme F420 autofluorescence, have been located in several microhabitats within the hindgut. Depending on the termite species, these organisms can be associated either with the hindgut wall or with filamentous prokaryotes attached to the latter, or they can occur as ectosymbionts or endosymbionts of certain intestinal flagellates.
FIG. 5 illustrates annual emissions of methane and carbon dioxide in the atmosphere by termites as calculated by various researchers. The annual emission rates of methane and carbon dioxide were estimated by researchers using the equation P=CΣi=1nAiBiFi where, P is the annual emission of the trace gas (in grams), Ai is the area of an ecological region (in square meters), Bi is the biomass of termites in that region (in grams per square per square meter) and Fi is the flux of the trace gas (in grams of gas per grams of termites per hour).
FIG. 6 illustrates a termites life cycle. As a xylophagous termite grows and develops, methanogens clearly play an integral role in the reproduction, growth, development and overall activity of the organism. The microbes play similar roles in the life-cycles of other wood-boring insects and cellulose consumers such as xylophagous beetles.
A series of termite control methods have been implemented historically with varying measurements of success. A brief description of those techniques is presented below.
Fumigation: Fumigation (“tenting”) has been the only method used for over forty years which insures complete eradication of all drywood termites from a structure. The phase-out of methyl bromide in the U.S. has positioned sulfuryl fluoride as the leading gas fumigant. Fumigation is a highly technical procedure which involves surrounding the structure with a gas-tight tarpaulin, releasing the gas inside the seal, and aerating the fumigant after a set exposure time.
Heat: Heat treatments are used to eradicate drywood termites. During the heat treatment the infested area is cordoned off with polyethylene or vinyl sheets. Temperature probes are placed in the hardest-to-heat locations and heat is applied with a high-output propane heater. After a lethal target temperature is achieved, the area can be cooled quickly.
Cold: Excessive cold is primarily applied by using liquid nitrogen, which is pumped into the targeted area until the temperature drops to a level lethal to drywood termites. Temperature probes are used to insure that lethal temperatures are attained.
Wood Injection: Wood injection or “drill-and-treat” applications have been used since the 1920s to treat drywood termite infestations which are accessible and detectable. An insecticide is injected into small holes drilled through any wood surface into termite galleries delivering the treatment directly to the pest population. This is the simplest and most direct method of treatment. The amount of drilling required and the effectiveness of this treatment depend on the chemical used and the nature of the infestation. Most chemicals will remain active in the wood after treatment to thwart resurgent colonies.
Borates: Spray and foam applications of products containing boron salts are applied to raw, uncoated wood surfaces. Because penetration depths of borate solutions and depth of drywood termite galleries vary, injections into existing infestations are usually being performed.
Microwave: Microwave energy, applied to relatively small sections of infested wood, kills termites by heating them. Thermocouples are inserted into treated members to ensure that adequate microwave energy is delivered.
Electrocution: The probe of a hand-held “gun” is passed slowly over the infested wood surface and inserted directly into pellet “kick-out” holes. The high voltage and low current energy emitted by the probe electrocutes termites in the immediate application area. There is no way to measure a lethal dose at a given location in wood with this device. In some cases, holes must be drilled into wood and wires inserted to improve penetration.
The most common delivery methods for the targeted treatment methods are baits and sprays. The baits consist of paper, cardboard, or other palatable food, combined with a slow-acting substance lethal to termites. The bait must be “tasty” enough that termites will readily consume it, even in the presence of competing tree roots, stumps, woodpiles and structural wood.
If the bait kills too quickly, sick or dead termites may accumulate in the vicinity of the bait stations, increasing the chance of avoidance by other termites in the area. Delayed-action also enhances transmission of the lethal agent to other termites, including those that never fed on the bait. Entire colonies can be eliminated in this manner, although total colony elimination is not always necessary to afford structural protection. The lethal compounds could also be made into a spray for use on susceptible wood surfaces or surfaces exhibiting infestation where pests need to be controlled. It could also be used incorporated into a sugar solution.